We are working with 57 histocompatibility (H-2 and non-H-2) mutations in the mouse, using the C57BL/6Kh and BALB/cKh strains. Seventeen of these mutations have been established as homozygous congenic or coisogenic lines, of which nine are in the H-2 complex. Forty more lines are under development, of which at least seven involve H-2. In addition, eleven suspected mutants are under test. Besides the establishment of the lines, our objectives include mapping the mutations, contributing to their initial characterization (especially in collaboration with others), and subsequently making such lines available to other laboratories for further study.